a. Field of the Invention
The present invention relates to the production of fluoride glass fibers, and more particularly, to the etching of preforms therefor.
b. Description of the Prior Art
Recently, there has been a great demand for optical fibers which operate in the mid-infrared. Mid-infrared fiber applications include ultra-low loss data communication links, temperature sensors, remote spectroscopy and powder delivery. Optical fibers drawn from fluoride glasses are most suitable for these applications because of their high optical transparency in the 2 to 5 micron wavelength region, as illustrated in FIG. 1. A typical fluoride glass contains between 50-70% ZrF.sub.4 which serves as the glass network modifier, and more additional fluorides such as AlF.sub.3, LaF.sub.3, NaF, LiF and PbF.sub.2 which are considered to be the glass secondary network modifiers. The typical fluoride glass compositions are given in U.S. Pat. No. 4,4445, 755, U.S. Pat. No. 4,674,835 and U.S. Pat. No. 4,141,741. Optical fibers with low optical attenuation are drawn from fluoride glass rods prepared by casting fluoride melts into cylindrical metallic molds. These fibers are now commercially being produced.
A limitation concerning commercial fluoride glass fibers at the present time is that they are mechanically weak. The low tensile strength of these fibers arises from flaws and defects which are induced on the surface of the rod in contact with the metallic mold. Efforst in strengthening fluoride glass fibers include mechanical polishing and chemical etching to remove surface flaws. The best result was obtained by Schneider et al., to be published in SPIE Proceedings, Vol. 799, using ZrOCl.sub.2 as an etching solution. Fluoride glass fibers drawn from glass rods etched with the ZrOCl.sub.2 solution exhibit an average tensile strength of 400 MPa. At this strength level, the use of fluoride glass fibers are limited to applications which do not require a small fiber bending radius. This would exclude medical applications such as general laser surgery, angioplasty, or blood gas analysis whose fiber bending requirements are very stringent. Attempts to further increase the fiber strength using ZrOCl.sub.2 have failed because of a consistent appearance of unknown submicroscopic defects deposited on the etched surface.